Envelope insertion or stuffing machines are used for inserting stacks of advertising material into envelopes. The individual inserts are moved from respective insert bins onto a conveyor which is advanced in a step-by-step manner to form insert stacks. The insert stacks are moved from the stepping conveyor onto a stuffing station. Envelopes are fed from an envelope bin onto the stuffing station where a stuffing mechanism opens the envelopes and feeds the insert stacks into the open envelopes. The stuffed envelopes are collected, labeled, stacked and generally bundled in numbered stacks according to zip code numbers. The operator must check the envelopes to verify the end of a zip code and the beginning of a new zip code on the envelopes.